The invention relates to light based range sensors, usually laser based range sensors, that make high precision optical measurements of surfaces having varying profiles and varying degrees of reflectivity. More specifically, the invention relates to non-contact sensors that optically measure the distance from the sensor head to the surface under test.
High-precision surface-profiling devices of the past have been primarily of the contact type. A spherical ruby stylus (of radius typically 1 mm) is used to probe the surface under test. In some systems, the deflection of the probe is measured by the closure of a mechanical microswitch; in other systems, the deflection is measured capacitively. Although such probes can be exceedingly accurate on suitable materials, the range of suitable materials is limited. Inaccurate readings or damage may occur if the sample is unsuitable. Examples of unsuitable samples include: soft, liquid, or sticky materials, very thin samples that would deform under stylus pressure, samples with concavities of radius smaller than the tip radius, or samples that are hazardous to touch. Additionally, in order to minimize probe and sample wear, the speed of measurement must be made low.
As an alternative to the electro-mechanical controllers and ruby-tipped contact probes, several types of measurement systems have been developed using optical triangulation. U.S. Pat. No. 4,733,969 discloses a laser probe for determining distance from the probe to the test object by projecting laser light onto a target surface through a source lens. The light is reflected from the target surface through a receiving lens and directed onto a pair of detectors. Light falling on the first detector indicates that the sensor (or probe) is within range for making a measurement. Light falling equally on, or in a predetermined proportion on, the two detectors indicates that a trigger point is reached and a coordinate measurement should be taken. Light falling off the second detector indicates that the sensor is no longer in range for measurements. The probe is able to focus the laser beam to approximately 0.001 inches. Alternatively, a cylindrical lens may be incorporated to project a stripe pattern onto the target object. The laser probe may be produced in a compact configuration using a reflecting mirror and additional focusing lenses. The advantages of this laser probe include the ability to integrate with existing coordinate measuring machines, the production of a very small spot size for measurement of very small and detailed objects, and a probe response speed that is fast and accurate. Unfortunately, this device only provides a binary comparison of distance against a pre-determined reference, whereas a numerical range reading would be more generally useful.
U.S. Pat. Nos. 4,891,772 and 4,872,747 describe a point and line range sensor that produces a numerical range reading based on optical triangulation. Once again, laser light is directed onto a target surface. The light reflected from the target surface is passed through a collimating lens to create a spot of light on a suitable. multi-element detector, and the position of the spot of light is analyzed.
A key feature of the apparatus disclosed in U.S. Pat. Nos.4,891,772 and 4,872,747 is the use of prisms to produce anamorphic magnification. This technique makes the instrument much more compact, while allowing the light to remain concentrated, which improves the performance of the instrument. After the returning light has passed through the collimating lens, it is directed toward the roughly-perpendicular face of a prism. The light exits the prism at a steep angle providing large angular magnification. A focusing lens directs the exiting collimated light onto the surface of a detector and a range measurement is determined. Alternatively, and preferably, two prisms are used, each providing approximately equal magnification. This scheme provides better performance than a single prism, since the dispersion and distortion from the first prism can be largely canceled by opposite dispersion and distortion from the second prism. The second prism may be oriented to provide total internal reflection, which can further reduce package size. The advantages of this system include the avoidance of contact with the target object, a small package size and the ability to maintain post-magnification light levels at substantially higher levels than non-anamorphic systems.
At least one limitation of the apparatus of U.S. Pat. Nos. 4,891,772 and 4,872,747 is that the anamorphic magnification results in considerable field-dependent astigmatism. This causes the spot on the detector to widen toward the ends of the working range of the sensor, which increases the uncertainty of the spot location. The astigmatism arises from fundamental characteristics of image formation from tilted planes, and cannot be reduced by any simple optimization of surface radii and tilts.
U.S. Pat. No. 5,633,721 describes a surface-position detection apparatus that uses optical triangulation to measure the location of a pattern of light directed onto a target surface. The return light from the pattern is collected and analyzed by a receiving optical system, consisting of a focusing lens, a prism or grating, a relay lens, and a spatially-sensitive detector. The focusing lens forms an image of the pattern on the surface of the prism or grating, and this image is re-focused onto the detector by the relay lens. The target surface, the focusing lens, and the front prism surface are tilted so as to satisfy the Scheimpflug condition. The prism serves to refract the image of the pattern and to reduce the obliquity of the final image.
Limitations on the technology disclosed in U.S. Pat. No. 5,633,721 relate to the prism. The front surface of the prism lies in an image plane and as such, any dust or surface defects on the prism appear in sharp focus on the detector, and can easily cause objectionable image degradation. Additionally, because the image is formed on the front surface of the prism an additional optical device (a relay lens) must be included, which adds to the bulk, weight and complexity in assembling the sensor system.
Moreover, each of the above patents faces the problem of limited range resolution. A fundamental limit on range resolution is imposed by the presence of speckle. Speckle is a well-known interference phenomenon observed with coherent sources, where irregularities in a surface placed in the optical path give rise to a characteristic granular spatial variation in the light intensity. Since this granularity persists despite focusing of the light, it places a fundamental limit on the resolution of laser triangulation sensors. The phenomenon of speckle, as it relates to optical triangulation, was analyzed in a paper entitled xe2x80x9cLaser Triangulation; Fundamental Uncertainty in Distance Measurement by Rainer G. Dorsch, Gerd Hxc3xa4usler, and Jxc3xcrgen Herrmann (Applied Optics, Vol. 33 (1994) pp. 1306-1314). The conclusion reached in the analysis was that, in order to reduce or eliminate the phenomenon of speckle, the object-space numerical aperture of the receiver lens must be maximized for optimal performance. The above-described surface measurement system patents did not address the maximization of numerical aperture. Due to neglect of this important factor, earlier sensors had limited range-to-resolution ratios, typically in the range of only 400:1.
Further, each of the above-described patents is only able to take and process a single surface measurement at a time. In other words, each of the above-described optical systems is designed so that the detector may detect only one spot of light, that is reflected from the target surface at a time.
In view of the above, there is a need for an optical sensor system that can produce precise measurements by reducing speckle through numerical aperture adjustment, that has improved range-to-resolution and range-to-accuracy ratios and that can determine multiple spots of light on the detector, which would enable it to make thickness measurements of transparent objects.
The problems described above are in large measure solved by the digital range sensor of the present invention. The digital range sensor comprises a light source, an optical element to focus the light from the source down to a small spot on a target, a second optical element that is mounted obliquely from the source axis, and a prism mounted on a multi-element detector, which in turn is mounted at the focus of the light returning from the target. The purpose of the prism on the detector is to direct the light onto the active surface of the detector at an angle closer to normal incidence than would otherwise be possible. The detector circuitry produces digital data which is transferred to a control module for processing and for producing a range measurement in millimeters.
It is an object and advantage of the present invention to produce a thickness measurement by the use of a single sensor and dynamic threshold control.
It is an object and advantage of the present invention to use a larger object-space numerical aperture than other comparable sensors, which reduces speckle size and reduces uncertainty in the range of measurement.
It is an object and advantage of the present invention to provide an improved range-to-resolution ratio on the order of 2000:1, compared to previous systems having a ratio of 400:1 as well as an improved range-to-accuracy ratio on the order of 200:1. With these improved ratios, the sensor of the present invention is more versatile than previous sensors, having a longer working range, and as such, reduces the number of sensors a customer must buy.
It is yet another object and advantage of the present invention to enable the sensor to communicate with a digital signal processor through use of a digital signal processing (DSP) serial port.
It is still another object and advantage of the present invention to provide a sensor that is flippable/symmetrical allowing for alternative mounting choices as well as stackable allowing for stacking similar and/or different sensors. Further, the sensor housing preferably incorporates precision mounting holes, pin and mount locating features, for repeatability in mounting.
It is another object and advantage of the present invention to produce a sensor having lower power dissipation than previously achieved.
Other objects and advantages of the present invention include: (1) storing calibration information within the sensor itself such that when the sensor is connected to a control module, the control module may detect the type of sensor present, thereby allowing easy interchangeability of sensors with a single controller card; (2) the ability of the control module to connect directly into an ISA bus of an IBM-compatible computer, allowing for faster data transfer than that of a previously known and used RS-232 interface.